1. Field of the Invention
The present invention relates to a control method for a hydraulic type continuously variable transmission such that a high-pressure oil passage and a low-pressure oil passage are formed between a hydraulic pump and a hydraulic motor.
2. Description of Background Art
A hydraulic type continuously variable transmission is known as a continuously variable transmission for use in a vehicle such as a motorcycle (See, for example, Japanese Patent Publication No. Hei 7-56338, Japanese Patent Publication No. Hei 8-06797 and Japanese Patent Laid-open No. 2006-200727). FIG. 1 shows a hydraulic type continuously variable transmission 10 (which will be hereinafter referred to also as “continuously variable transmission 10”) in the existing art (cited from FIG. 9 of Japanese Patent Laid-open No. 2006-200727). In the hydraulic type continuously variable transmission 10, torque from an engine 14 is input to a hydraulic pump 12. In the hydraulic pump 12, the input torque is converted into an oil pressure of a working fluid 22 by using an input shaft 16, a pump swash plate 18, and a pump plunger 20. The working fluid 22 is transmitted through an annular high-pressure oil passage 26 in a hydraulic closed circuit 24 to a hydraulic motor 28. In the hydraulic motor 28, the oil pressure of the working fluid 22 is reconverted into torque by using a motor plunger 30, a motor swash plate 32, and an output shaft 34, and the torque is output from the output shaft 34.
The magnitude of the torque to be transmitted from the input shaft 16 to the output shaft 34 can be changed by using a shifting actuator 36 to adjust the angle of the motor swash plate 32 in the hydraulic motor 28. In other words, the ratio between the input torque of the hydraulic pump 12 and the output torque of the hydraulic motor 28 (which will be hereinafter referred to also as “speed ratio R”) can be adjusted by adjusting the angle of the motor swash plate 32. The working fluid 22 transmitted from the hydraulic pump 12 to the hydraulic motor 28 is returned to the hydraulic pump 12 through an annular low-pressure oil passage 38 in the hydraulic closed circuit 24 for the purpose of reuse.
The present inventor has studied to find that when reacceleration is performed just after reduction of an actual engine speed NE [rpm] (i.e., when a target engine speed T_NE [rpm] is rapidly increased just after reduction of a vehicle speed V [km/h]), the actual engine speed NE temporarily becomes much greater than the target engine speed T_NE as shown in FIG. 7. This point has been further studied to estimate the pressure of the following cause.
When the vehicle is accelerated (i.e., when the rotational speed of the input shaft 16 of the hydraulic pump 12 is increased), torque transmission is performed from the hydraulic pump 12 to the hydraulic motor 28 in the hydraulic type continuously variable transmission 10, so that the oil pressure of the working fluid 22 in the high-pressure oil passage 26 becomes relatively high and the oil pressure of the working fluid 22 in the low-pressure oil passage 38 becomes relatively low. In contrast, when the vehicle is decelerated (i.e., when the rotational speed of the input shaft 16 of the hydraulic pump 12 is decreased), torque transmission from the hydraulic pump 12 to the hydraulic motor 28 is not performed (conversely, torque transmission from the hydraulic motor 28 to the hydraulic pump 12 is performed), so that the oil pressure of the working fluid 22 in the high-pressure oil passage 26 becomes relatively low and the oil pressure of the working fluid 22 in the low-pressure oil passage 38 becomes relatively high. Accordingly, when the vehicle is reaccelerated just after deceleration, both the oil pressure of the working fluid 22 in the high-pressure oil passage 26 and the oil pressure of the working fluid 22 in the low-pressure oil passage 38 temporarily become relatively high, causing a reduction in resistance in torque transmission from the hydraulic pump 12 to the hydraulic motor 28. As a result, the actual engine speed NE (the actual rotational speed of the input shaft 16 of the hydraulic pump 12) becomes much greater than the target engine speed T_NE (the target rotational speed of the input shaft 16) as mentioned above. Further, when the engine speed NE is increased, the angle of the motor swash plate 32 is adjusted so as to increase the speed ratio R in general (i.e., so as to obtain an effect similar to that obtained by upshifting). However, as shown in FIG. 7, a rapid increase in engine speed NE causes an unexpected increase in speed ratio R (an unexpected decrease in output torque), so that desirable torque cannot be produced to cause a reduction in operability.